In OTL, small groups of Askhenazi Jews immigrated to the Ottoman Empire between 1421–1453. Among these new Ashkenazi immigrants was Rabbi Yisak Sarfati who became the Chief Rabbi of
Edirne and wrote a letter inviting the European Jewry to settle in the Ottoman Empire, in which he stated that: "Turkey is a land wherein nothing is lacking" and asking: "Is it not better for you to live under Muslims than under Christians?".
Many had taken the Rabbi up on his offer, including the Jews expelled from the German Duchy of Bavaria by Duke Louis IX in 1470. These Ashkenazi Jews quickly diluted into the already large Romaniot (Byzantine) Jewish communities that had become part of the Ottoman Empire as they had conquered lands from the Byzantine Empire.
The next major Jewish immigration to the Ottoman Empire will be the Sephardic Jews expelled from Spain by the 1492 Alhambra decree (200,000). This General Edict on the Expulsion of the Jews also expelled the Jews from Sicily (1493, approx. 37,000), from Portugal (1496) and from Calabria in 1554. The Sephardic Jews eclipsed and absorbed the Romaniot and Askhenazi communities profoundly changing the culture and structure of Jewish communities in the Ottoman Empire. For example, Egypt received a large number of the exiles, who soon out-numbered the pre-existing Musta’arabi, similarly to other smaller Jewish communities in the Empire.
I know I am partially cheating, but Yiddish is a High-German based language. A larger Askhenazi immigration to the Ottoman Empire, following Rabbi Yisak Sarfati’s letter, seems to be a nice solution to create a Germanic-arabic vernacular. Of course, you need to separate them geographically and socially from the later Sephardic migrants to avoid an assimilation or you can find a way to outnumber them. The later method is hard, as you can’t easily find more than 200’000 Askhenazi Jews to be expelled en bloc, as they were far less numerous...
POD : I think that the example of the Alhambra decree and the use of the letter of Yisak Sarfati to demonize the Jews as Ottoman agents could lead to larger persecutions against the Askhenazi communities, especially in Germany and Poland. The persecuted Askhenazi communities and their survivors would have to flee to the south, as France and Spain were already persecuting them. Migrating to the Ottoman Empire would ben seen as their only solution to survive.
Of course, some Askhenazim will stay in Germany and Poland, where they will survive and continue to develop their original versions of Yiddish.
While the Sepharadim were mainly relocated to major cities in OTL, the majority of the Askhenazim would be relocated to the countryside and smaller settlements, as most of them would be peasants, artisans and often poorer than the Sepharadim. Only a minority of them would settle in major cities and be absorbed by the Sepharadim as in OTL. Later, they will be selected by the Sultan Selim I and his successor Suleiman I to settle and pacify the newly conquered peripheral territories in the Arabic peninsula and Iraq, creating urban (and later military) Askhenazi communities in the region, administrated by Askhenazi rabbinim and councils. In order to differentiate those communities from the European Askhenazi communities, the use of the ethnonyme Alamanim is recommenced by most specialists, which is why we will use it.
Politically and socially, the Alamanim quicklly thrived thanks to their status of middle men between the Ottoman Empire and the local sheikdoms. Unfortunately this statut turned them into a symbol of the Ottoman authority and let them vulnerable during early Arab revolts. After suffering from those repeated anti-Jewish/Ottoman revolts in the late 16th century, male Alamanim were allowed to bear weapons in exchange of military service, acting as a professional military force for the Ottomans.
Despite this, the influence of Alamanim intellectuals and philosophers on Arab nationalism shouldn’t be undervalued. Their nationalistic and militaristic vision of proto-sionism and European-based ideologies, such as anarchism and communism, greatly shaped modern Arab nations.
The Alamanim weren’t victims of the Shoah and escaped from the European pogroms of the 18th-19th centuries. This historical luck does explain why the Alamanim are now the biggest worldwide "Jewish" group. Nevertheless, they are still a minority in Israel.
The four biggest Alamani communities are still thriving in Medina, Basra, Baghdad and Damascus, despite the 1950’s and later migrations to Israel. Most Alamanim are still shunned in Israel due to their religious differences and their traditional acceptance of (Arab) converts, a bastardization for some of the most orthodox Israeli rabbinim. Many returning Alamanim later chose to migrate to Europe and the United States.
Religiously, contrary to Sepharadim, most sects of the Alamanim didn’t hesitate to proselytize and convert members of other religious communities, especially Sepharadim and Arabs, in order to safeguard their communities. The use of Yiddish and even Alamani during religious ceremonies, instead of Hebrew or Aramaic, is one of the common features of the Alamanim.
During the 18th century, various elements of Islam have also been appropriated by Alamani sects in contact with Wahhabism, like stricter modesty rules for women and their own version of the five pillars. It’s limited to the Arabic peninsula and Medina community.
The Alamani and their faith are nowadays religiously and culturally diverse. Even if it’s easy to distinguish between the most traditional or orthodox groups and the atheistic reformists due to their garments, it’s often harder to consider modern Alamanim as Europeans.
The social mixity and porosity between the local Arabs and the Askhenazi newcomers slowly developed the vernacular known as Alamani language, fused with elements taken from Yiddish and Arabic as well as minor traces and loanwords from Farsi and Kurdish dialects. This definition does explain why some linguists do consider the dialect of Alamani spoken in Tabriz as a proto-language, due to the massive influence of Farsi in its grammar structure and everyday vocabulary.
Alamani is written with a fully vocalized version of the Arabic alphabet.